The present invention relates to the field of hyperpolarized inert gases. More particularly, the present invention relates to a method for minimizing the loss of 129Xe polarization during the cryogenic accumulation process.
As is known in the art, polarized 129Xe must be separated from the 4He carrier gas. Due to the high freezing/melting point of Xe, about 160K, separation from the carrier gas is typically accomplished by freezing the Xe out of the mixture in liquid nitrogen.
Gatzke et al., in Phys Rev Lett 70, 690, 1993, have established that fields of greater than 500 Gauss are required to achieve the longest possible polarization relaxation time of solid 129Xe of about 2-3 hours at 77K. Cates et al., in Phys Rev Lett 65, 2591, 1990, determined that, when frozen in liquid N2 at 77K in a sealed Pyrex cell, 129Xe relaxation rates are essentially independent of magnetic field strength above 500G. In view of these findings, polarizers today position permanent magnets providing a field strength of about 500G around solid 129Xe.
However, further investigation supporting the present invention has revealed that this field strength, while sufficient for minimizing relaxation at 77K, is insufficient for minimizing relaxation at temperatures closer to the melting point. The relaxation rate of the solid 129Xe can be very fast near its freezing point, limiting the amount of 129Xe polarization which is retained throughout the accumulation process. Presently, a factor of 2 or more in polarization is lost in the freezing and thawing process.
There is therefore a need for a system and method of reducing nuclear-spin relaxation rates when melting frozen 129Xe.
In view of the needs of the art, the present invention provides a yoke supporting a magnet which provides a magnetic field of at least 2 kiloGauss about frozen 129Xe which is to be melted and collected.
The yoke of the present invention supports a magnet about the entire region of a collector where frozen 129Xe accumulates. The yoke thereby provides a holding magnetic field which extends through the volume of 129Xe accumulation. The collector is desirably constructed in the form of a xe2x80x98cold fingerxe2x80x99 as is known in the art. The holding magnetic field may be induced by permanent magnets, electromagnets, superconducting magnets, or standard magnets. The holding field may further be provided by a yoke body which couples the magnetic field of the magnets about the cold finger.
The present invention further provides a method for reducing the relaxation rate of accumulated polarized 129Xe in a chamber. The method includes the step of positioning the chamber in a magnetic holding field having a magnetic field strength of greater than 2 kiloGauss. The method further maintains the accumulated 129Xe in the holding field as the accumulated 129Xe is thawed and subsequently recrystallized.